Focal adhesions: a nexus for intracellular signaling and cytoskeletal dynamics

The Cdc42 and Rac1 GTPases are required for capillary lumen formation in three-dimensional extracellular matrices

Here we show a requirement for the Cdc42 and Rac1 GTPases in endothelial cell (EC) morphogenesis in three-dimensional extracellular matrices. Cdc42 and Rac1 specifically regulate EC intracellular vacuole and lumen formation in both collagen and fibrin matrices. Clostridium difficile toxin B(which blocks all three Rho GTPases) completely inhibited the ability of ECs to form both vacuoles and lumens, whereas C3 transferase, a selective inhibitor of Rho, did not. Expression of either dominant-negative (N17) or constitutively active (V12) Cdc42 using recombinant adenoviruses dramatically inhibited EC vacuole and lumen formation in both collagen and fibrin matrices. Both vacuole and lumen formation initiated in ECs expressing dominant-negative(N17) Rac1 but later collapsed, indicating a role for Rac1 during later stages of vessel development. Analysis of cultures using confocal microscopy revealed green fluorescent protein-V12Rac1, -Rac1 wild-type and -Cdc42 wild-type chimeric proteins targeted to intracellular vacuole membranes during the lumen formation process. Also, expression of the verprolin-cofilin-acidic domain of N-WASP, a downstream Cdc42 effector, in ECs completely interfered with vacuole and lumen formation. These results collectively reveal a novel role for Cdc42 and Rac1 in the process of EC vacuole and lumen formation in three-dimensional extracellular matrices.

Integrin alpha3beta1 (CD 49c/29) is a cellular receptor for Kaposi's sarcoma-associated herpesvirus (KSHV/HHV-8) entry into the target cells

Human herpesvirus-8 (HHV-8) is implicated in the pathogenesis of Kaposi's sarcoma. HHV-8 envelope glycoprotein B possesses the RGD motif known to interact with integrin molecules, and HHV-8 infectivity was inhibited by RGD peptides, antibodies against RGD-dependent alpha3 and beta1 integrins, and by soluble alpha3beta1 integrin. Expression of human alpha3 integrin increased the infectivity of virus for Chinese hamster ovary cells. Anti-gB antibodies immunoprecipitated the virus-alpha3 and -beta1 complexes, and virus binding studies suggest a role for alpha3beta1 in HHV-8 entry. Further, HHV-8 infection induced the integrin-mediated activation of focal adhesion kinase (FAK). These findings implicate a role for alpha3beta1 integrin and the associated signaling pathways in HHV-8 entry into the target cells.

Pax3 regulates morphogenetic cell behavior in vitro coincident with activation of a PCP/non-canonical Wnt-signaling cascade

Mutations to Pax3 and other Pax family genes in both mice and humans result in numerous tissue-specific morphological defects. Little is known, however, about the cellular and molecular mechanisms by which Pax genes regulate morphogenesis. We previously showed that Pax3 induces cell aggregation and a mesenchymal-to-epithelial transition in Saos-2 cells. We show here that Pax3-induced aggregates arise through the formation of distinct structures involving cell rearrangements and cell behaviors resembling those that occur during gastrulation and neurulation known as convergent extension. During these Pax3-induced processes, Dishevelled and Frizzled are localized to the actin cytoskeleton and both proteins coimmunoprecipitate focal adhesion components from detergent-insoluble cell fractions. We show further that these Pax3-induced cell movements are associated with activation of a Wnt-signaling cascade, resulting in induction and activation of c-Jun-N-terminal kinase/stress activated protein kinase (JNK/SAPK). All of these Wnt-signaling factors exhibit altered subcellular distribution in Pax3-expressing cells. In particular, we show the localization of JNK/SAPK to both the nucleus and to cytoplasmic multi-vesicular structures. These data show that Pax3 regulates morphogenetic cell behavior and that regulation of a conserved, planar cell polarity/noncanonical Wnt-signaling cascade entailing JNK activation is a function of Pax3 activity.

Role of integrins in cell invasion and migration

As cancer cells undergo metastasis--invasion and migration of a new tissue--they penetrate and attach to the target tissue's basal matrix. This allows the cancer cell to pull itself forward into the tissue. The attachment is mediated by cell-surface receptors known as integrins, which bind to components of the extracellular matrix. Integrins are crucial for cell invasion and migration, not only for physically tethering cells to the matrix, but also for sending and receiving molecular signals that regulate these processes.

Preferential initiation of PC12 neurites in directions of changing substrate adhesivity

When PC12 cells are grown on substrates showing a gradient of nonspecific adhesion, they preferentially initiate neurites in directions of changing adhesivity, whether that change is in the direction of increasing or decreasing adhesivity. This preference for changing adhesivity is ablated both by C. difficile toxin A, which inhibits all Rho-family GTPases, and by C. botulinum C3 exoenzyme, which specifically inhibits Rho.

Integrin control of cell cycle: a new role for ubiquitin ligase

Receptor tyrosine kinases and integrins are activated by growth factors and extracellular matrix, respectively. Their activation leads to signal transduction cascades that control many aspects of cell phenotype, including progression through the G(1) phase of the cell cycle. However, the signalling cassettes driven by growth factors and matrix do not work independently of each other. Integrin triggering is essential to facilitate kinase- and GTPase-mediated signals and thereby drive efficient transfer of information through the growth factor-cyclin axis. A recent study indicates that an additional type of player has a key role in adhesion-regulated control of cell cycle, namely ubiquitin ligase.

Goniodomin A, an antifungal polyether macrolide, exhibits antiangiogenic activities via inhibition of actin reorganization in endothelial cells

Goniodomin A (GDA) is an antifungal polyether macrolide isolated from the dinoflagellate Goniodoma pseudogoniaulax. Previous studies revealed that GDA profoundly affected cytoskeletal reorganization. We examined the effect of GDA on the angiogenic properties of vascular endothelial cells. GDA itself did not affect proliferation of, migration of, and tube formation in type I collagen gels by, bovine aortic endothelial cells (BAECs). Proliferation of BAECs stimulated by bFGF was not affected by GDA at concentrations of up to 10 nM. However, at similar concentrations, GDA significantly inhibited bFGF-induced migration and tube formation in type I collagen gels by BAECs. Actin reorganization is required for cell migration. GDA caused the perinuclear aggregation of filamentous actin and inhibited stress fiber formation in bFGF- or VEGF-stimulated BAECs and lysophosphatidic acid-stimulated HeLa cells. However, GDA did not affect stress fiber structures already formed through Gbetagamma expression or in constitutively active RhoA mutant HeLa cells. Finally, GDA inhibited forming of vasucular system in a chorioallantoic membrane. Our results indicated that GDA suppressed angiogenic properties of ECs at least in part through the inhibition of actin reorganization and inhibited angiogenesis in vivo.

The human papillomavirus E6 protein and its contribution to malignant progression

The human papillomavirus (HPV) E6 protein is one of three oncoproteins encoded by the virus. It has long been recognized as a potent oncogene and is intimately associated with the events that result in the malignant conversion of virally infected cells. In order to understand the mechanisms by which E6 contributes to the development of human malignancy many laboratories have focused their attention on identifying the cellular proteins with which E6 interacts. In this review we discuss these interactions in the light of their respective contributions to the malignant progression of HPV transformed cells.

Taking cell-matrix adhesions to the third dimension

Adhesions between fibroblastic cells and extracellular matrix have been studied extensively in vitro, but little is known about their in vivo counterparts. Here, we characterized the composition and function of adhesions in three-dimensional (3D) matrices derived from tissues or cell culture. "3D-matrix adhesions" differ from focal and fibrillar adhesions characterized on 2D substrates in their content of alpha5beta1 and alphavbeta3 integrins, paxillin, other cytoskeletal components, and tyrosine phosphorylation of focal adhesion kinase (FAK). Relative to 2D substrates, 3D-matrix interactions also display enhanced cell biological activities and narrowed integrin usage. These distinctive in vivo 3D-matrix adhesions differ in structure, localization, and function from classically described in vitro adhesions, and as such they may be more biologically relevant to living organisms.

Transmembrane crosstalk between the extracellular matrix--cytoskeleton crosstalk

Integrin-mediated cell adhesions provide dynamic, bidirectional links between the extracellular matrix and the cytoskeleton. Besides having central roles in cell migration and morphogenesis, focal adhesions and related structures convey information across the cell membrane, to regulate extracellular-matrix assembly, cell proliferation, differentiation, and death. This review describes integrin functions, mechanosensors, molecular switches and signal-transduction pathways activated and integrated by adhesion, with a unifying theme being the importance of local physical forces.

Phosphoinositides, key molecules for regulation of actin cytoskeletal organization and membrane traffic from the plasma membrane

Phosphoinositide plays a critical role not only in generating second messengers, such as inositol 1,4,5-trisphosphate and diacylglycerol, but also in modulating a variety of cellular functions including cytoskeletal organization and membrane trafficking. Many inositol lipid kinases and phosphatases appear to regulate the concentration of a variety of phosphoinositides in a specific area, thereby inducing spatial and temporal changes in their availability. For example, local concentration changes in phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) in response to extracellular stimuli cause the reorganization of actin filaments and a change in cell shape. PI(4,5)P(2) uncaps the barbed end of actin filaments and increases actin nucleation by modulating a variety of actin regulatory proteins, leading to de novo actin polymerization. PI(4,5)P(2) also plays a key role in membrane trafficking processes. In endocytosis, PI(4,5)P(2) targets clathrin-associated proteins to endocytic vesicles, leading to clathrin-coated pit formation. On the contrary, PI(4,5)P(2) must be dephosphorylated when they shed clathrin coats to fuse endosome. Thus, through regulating actin cytoskeleton organization and membrane trafficking, phosphoinositides play crucial roles in a variety of cell functions such as growth, polarity, movement, and pattern formation.

Regulation of Rho GTPases by p120-catenin

Three recent reports indicate that p120-catenin can modulate the activities of RhoA, Rac and Cdc42, suggesting an elegant and previously unexpected mechanism for regulating the balance between adhesive and motile cellular phenotypes. The observations in these reports provide important new clues toward p120's mechanism of action and provide a potential explanation for the metastatic phenotype exhibited in carcinoma cells that have lost E cadherin expression.

Assembly and mechanosensory function of focal contacts

Focal contacts, focal complexes and related extracellular matrix adhesions are used by cells to explore their environment. These sites act as mechanosensory 'devices', where internal contractile forces or externally applied force can regulate the assembly of the adhesion site and trigger adhesion-dependent signaling involving Rho-family small G-proteins and other signaling pathways. The molecular mechanisms underlying these processes are discussed.

Syndecan-4 and focal adhesion function

Two groups have now reported the viability of mice that lack syndecan-4. These mice have wound healing/angiogenesis problems, and fibroblasts from these animals differ in adhesion and migration from normal. This is consistent with recent in vitro data indicating a need for signaling via syndecan-4 for focal adhesion formation, and reports that overexpression of proteins that bind syndecan-4 can modify cell adhesion and migration.

Integrin tyrosine phosphorylation in platelet signaling

The beta 3 integrin cytoplasmic tyrosine (ICY) motif of alpha IIb beta 3 becomes tyrosine phosphorylated during platelet aggregation, causing Shc and myosin to interact with the beta-integrin cytoplasmic domain. Platelets from mice lacking beta 3 ICY motif tyrosines formed defective aggregates and poorly retracted clots, establishing integrin tyrosine phosphorylation as a key mediator of beta 3-integrin signals.

Paxillin-ARF GAP signaling and the cytoskeleton

Several new families of ARF GTPase activating proteins (ARF GAPs) have been described recently that associate with paxillin and other cytoskeletal and signaling proteins. Important insights have been gained regarding their subcellular distribution, enzymatic specificity and protein scaffold function. Evidence suggests an important role for ARF GAPs in mediating changes in the cell's actin cytoskeleton in response to adhesion and growth factor stimulation.

Intrinsic activating properties of GP IIb/IIIa blockers

Potential intrinsic activating properties are probably the most controversially discussed issues with respect to GP IIb/IIIa blockers, especially since clinical trials with oral GP IIb/IIIa blockers revealed disappointing results. Based on the finding that currently clinically used GP IIb/IIIa blockers are ligand mimetics, experimental data are discussed, demonstrating an intrinsic activating effect of ligand mimetic GP IIb/IIIa blockers that potentially results in fibrinogen binding to alpha(IIb)beta(3) and in platelet aggregation. Furthermore, the inhibitory effect of aspirin on GP IIb/IIIa blocker-induced platelet aggregation is discussed as a clinically relevant finding. Finally, the potential association of GP IIb/IIIa blocker-induced thrombocytopenia with platelet activation is described.

RhoA inactivation by p190RhoGAP regulates cell spreading and migration by promoting membrane protrusion and polarity

The binding of extracellular matrix proteins to integrins triggers rearrangements in the actin cytoskeleton by regulating the Rho family of small GTPases. The signaling events that mediate changes in the activity of Rho proteins in response to the extracellular matrix remain largely unknown. We have demonstrated in previous studies that integrin signaling transiently suppresses RhoA activity through stimulation of p190RhoGAP. Here, we investigated the biological significance of adhesion-dependent RhoA inactivation by manipulating p190RhoGAP signaling in Rat1 fibroblasts. The inhibition of RhoA activity that is induced transiently by adhesion was antagonized by expression of dominant negative p190RhoGAP. This resulted in impaired cell spreading on a fibronectin substrate, reduced cell protrusion, and premature assembly of stress fibers. Conversely, overexpression of p190RhoGAP augmented cell spreading. Dominant negative p190RhoGAP elevated RhoA activity in cells on fibronectin and inhibited migration, whereas overexpression of the wild-type GAP decreased RhoA activity, promoted the formation of membrane protrusions, and enhanced motility. Cells expressing dominant negative p190RhoGAP, but not control cells or cells overexpressing the wild-type GAP, were unable to establish polarity in the direction of migration. Taken together, these data demonstrate that integrin-triggered RhoA inhibition by p190RhoGAP enhances spreading and migration by regulating cell protrusion and polarity.

Myosin IIB is required for growth cone motility

Growth cones are required for the forward advancement and navigation of growing axons. Modulation of growth cone shape and reorientation of the neurite are responsible for the change of outgrowth direction that underlies navigation. Change of shape involves the reordering of the cytoskeleton. Reorientation of the neurite requires the generation of tension, which is supplied by the ability of the growth cone to crawl on a substrate. The specific molecular mechanisms responsible for these activities are unknown but are thought to involve actomyosin-generated force combined with linkage to the cell surface receptors that are responsible for adhesion (Heidemann and Buxbaum, 1998). To test whether myosin IIB is responsible for the force generation, we quantified shape dynamics and filopodial-mediated traction force in growth cones from myosin IIB knock-out (KO) mice and compared them with neurons from normal littermates. Growth cones from the KO mice spread less, showed alterations in shape dynamics and actin organization, and had reduced filopodial-mediated traction force. Although peak traction forces produced by filopodia of KO cones were decreased significantly, KO filopodia occasionally developed forces equivalent to those in the wild type. This indicates that other myosins participate in filopodial-dependent traction force. Therefore, myosin IIB is necessary for normal growth cone spreading and the modulation of shape and traction force but acts in combination with other myosins for some or all of these activities. These activities are essential for growth cone forward advancement, which is necessary for outgrowth. Thus outgrowth is slowed, but not eliminated, in neurons from the myosin IIB KO mice.

RhoA is required for monocyte tail retraction during transendothelial migration

Transendothelial migration of monocytes is the process by which monocytes leave the circulatory system and extravasate through the endothelial lining of the blood vessel wall and enter the underlying tissue. Transmigration requires coordination of alterations in cell shape and adhesive properties that are mediated by cytoskeletal dynamics. We have analyzed the function of RhoA in the cytoskeletal reorganizations that occur during transmigration. By loading monocytes with C3, an inhibitor of RhoA, we found that RhoA was required for transendothelial migration. We then examined individual steps of transmigration to explore the requirement for RhoA in extravasation. Our studies showed that RhoA was not required for monocyte attachment to the endothelium nor subsequent spreading of the monocyte on the endothelial surface. Time-lapse video microscopy analysis revealed that C3-loaded monocytes also had significant forward crawling movement on the endothelial monolayer and were able to invade between neighboring endothelial cells. However, RhoA was required to retract the tail of the migrating monocyte and complete diapedesis. We also demonstrate that p160ROCK, a serine/threonine kinase effector of RhoA, is both necessary and sufficient for RhoA-mediated tail retraction. Finally, we find that p160ROCK signaling negatively regulates integrin adhesions and that inhibition of RhoA results in an accumulation of beta2 integrin in the unretracted tails.